To perform satisfactorily, the alloys that are used in a number of demanding applications such as screws and barrels in the plastic injection molding industry, must be resistant to wear and corrosive attack. The trend in the industry is to keep increasing processing parameters (e.g., temperature and pressure), which in turn impose ever-increasing demands on the alloys and their ability to successfully withstand corrosive attack and wear by the materials being processed. In addition, the corrosiveness and abrasiveness of those materials are constantly increasing.
In order to withstand the stresses imposed during operation, the tool steel must also possess sufficient mechanical properties, such as hardness, bend fracture strength, and toughness. In addition, the tool steel must possess sufficient hot workability, machinability and grindability to ensure that parts with the required shape and dimensions can be manufactured.
The corrosion resistance of wear resistant tool steels depends primarily on the amount of “free” chromium in the matrix, i.e., the amount of chromium that is not “tied up” into carbides. Due to the formation of chromium-rich carbides, the amount of “free” chromium in the matrix is not necessarily the same amount as that in the overall chemical composition. For good corrosion resistance, through-hardening tool steels must contain at least 12 wt. % of “free” chromium in the martensitic matrix after heat treatment.
The wear resistance of tool steels depends on the amount, type, and size distribution of the primary carbides, as well as the overall hardness. The main function of the primary alloy carbides, due to their high hardness, is to provide wear resistance. Of all types of primary carbides commonly found in tool steels, vanadium-rich MC primary carbides possess the highest hardness. In general, the higher the volume fraction of primary carbides, the higher the wear resistance of the tool steel, and the lower its toughness and hot workability.
Corrosion and wear resistant martensitic tool steels must also contain a relatively high level of carbon for the formation of primary carbides and heat treatment response. As chromium has a high affinity for carbon with which it forms chromium-rich carbides, corrosion and wear resistant tool steels must contain excess chromium over the amount necessary for corrosion resistance to allow for carbide formation.
The corrosion and wear resistant martensitic tool steels that are commercially available include grades such as 440C, CPM S90V, M390, Elmax and HTM X235, among others. Despite the fact that the overall chromium content of some of these alloys is as high as 20 wt. % (e.g., M390), the corrosion resistance is not necessarily as good as one might expect. Depending on the overall chemical composition and the heat treatment parameters, a large amount of chromium is pulled out of the matrix and tied up into chromium-rich carbides. This tied up chromium does not contribute toward corrosion resistance.
One of the practices that has been used to improve the combination of wear and corrosion resistance, as exemplified by U.S. Pat. No. 2,716,077, is to add vanadium. This alloying addition forms hard vanadium-rich MC primary carbides and ties up a part of the carbon. Due to the fact that the affinity of vanadium toward carbon is higher than that of chromium, the presence of vanadium in tool steels decreases the amount of chromium-rich primary carbides, all other conditions being equal (i.e., the overall chromium and carbon content and the heat treatment parameters).
The corrosion resistance of tool steels is further improved by the presence of molybdenum in the martensitic matrix. An example is Crucible 154 CM grade, which is based on the Fe-1.05C-14Cr-4Mo system.
A primary objective of the invention is to provide a wear and corrosion resistant powder metallurgy tool steel with significantly improved corrosion and wear resistance. In the alloy of the invention, in addition to vanadium, niobium is used to further increase the amount of MC primary carbides. This in turn decreases the amount of chromium-rich primary carbides due to the fact that niobium has an even higher affinity toward carbon than vanadium.
To obtain the desired combination of wear and corrosion resistance in the alloy of the invention it is necessary to have chromium in combination with niobium, molybdenum, and vanadium within the claimed ranges. Specifically, the presence of niobium within the claimed range lowers the amount of chromium that dissolves in the MC primary carbides and thus increases the amount of “free” chromium in the matrix. Niobium retards the formation of chromium-rich carbides, enabling a greater part of the chromium to remain in the matrix to achieve the desired corrosion resistance of the alloy. Thus, balancing the chromium, niobium, and vanadium contents within the claimed limits allows the excess chromium (over that combining with the carbon to form carbides) to remain in the matrix to provide the desired corrosion resistance. Vanadium and niobium are added to achieve directly wear resistance, and to indirectly improve corrosion resistance.